A slim well is highly desirable since it reduces the costs for drilling and completion substantially. Such a well should be designed with the smallest possible diameter needed. Slim hole drilling has been used onshore for a long time. A limited application of this technique has been used in offshore applications from a floating vessel. Slim hole drilling offers a significant potential in reduction of drill cuttings discharge, reduced volume of drilling fluids, cement, casing string weight, etc. One of the main limitations when drilling in deep waters from floating drilling vessels is the size and the weight of the marine drilling riser. A slim hole allows reduction of the size and the weight of the riser. However, due to the close distance between the pore pressure curve and the fracturing curve, relatively many casing points are normally required to reach the reservoir section. So, even with conventional slim hole drilling, the weight and the size of the marine drilling riser will be significant and require a relatively costly drilling vessel to be used.
Normally, riserless drilling takes place down to the setting point for, e.g., a 20″ surface casing, typically 800 m below mudline (BML). Riserless drilling in this context means that the drill string is not enclosed within a tube or riser. Since at this depth the risk of encountering a formation containing fluids and/or gas that may escape is increasing from this point, most deep water drilling systems are based on using a standard 18¾″ wellhead, a 18¾″ BOP and a 21″ marine drilling riser. If fluids and/or gas should escape from the well bore, these will flow into the drilling riser and not pollute the seawater. The standard system is hereafter termed 18¾″ wellhead system. Through the system, comprising the drilling riser, the BOP and the wellhead, the casings will be installed. As the second stage of the well bore normally a hole with a size to receive a 13⅜″ casing will be drilled. Then a third stage with a hole to receive a 9⅝″ casing will be drilled and subsequently a fourth stage to receive a 7″ liner will be drilled. Finally a 7″ tie-back string for production may be installed. Logging, coring and well testing will normally be performed in a 8½″ open hole section below the 9⅝″ casings.
Today, a 4¾″ open hole through the reservoir section is sufficient for application of standard tools for logging, coring and well testing equipment, etc.
The problem of applying slim hole drilling on deep wells is that there is a limit on how long each section of casing reasonably can be. This puts a limitation on how deep wells that can be drilled using this technique.